One particularly infamous example of this is the Butterfly's Sting build. You have one person play TWF with kukris and that feat, and another player has a big 2H weapon with a 4x crit like a scythe.

The kukri user is built for accuracy above all, trying to get as many hits in as possible in order to get those crit confirms; these are then passed on to the scythe wielder who gets an automatic crit and likely annihilates whatever they were targeting. It can be pretty nutty once Improved Critical comes online and you're getting those crits consistently.

Another fun one is a Skald paired with a caster focused around summoning; the Skald wants to take a Linnorm Death Curse rage power. These are normally useless to player characters since they require someone to die for them to do anything, but in the hands of a Skald buffing summons then you are quite happy to make killing your summons a rather uncomfortable proposition for your enemies.

Another fun one is a Skald paired with a caster focused around summoning; the Skald wants to take a Linnorm Death Curse rage power. These are normally useless to player characters since they require someone to die for them to do anything, but in the hands of a Skald buffing summons then you are quite happy to make killing your summons a rather uncomfortable proposition for your enemies.

To expound on this: Use a Half-Orc Monster Tactician Inquisitor with the Rage domain (or Anger Inquisition) and the Amplified Rage feat. Bonus points if you decide to get Augment Summoning. Suddenly, you have raging monstrosities that seriously hurt the enemy and it was even worse when they died.

One of my favorite combos was when my wife and I were playing in a Strange Aeons game. She was a Bloodrager and I was a Shaman. We both took Bonded Minds and I took Share Spells. The ensuing awesomeness of me being able to cast any personal buff spell on her (think Fey Form and the like) was quite enjoyable.

I actually have a pair of characters built to be played together. One is a Phantom Thief Rogue and the other is a Dashing Thief Swashbuckler. They're focused a lot on stealing and disarming, and Phantom has Butterfly's Sting to give crits to Dashing. They may not be too interesting or unique but I am hoping to play them at some point.

The idea is this:
1. Riving strike makes enemies hit with arcane strike take a -2 to saves vs. spells for a turn.
2. Cornurgon smash (or bludgeoner/enforcer) allows you to intimidate as a free action when you hit with power attack. The shaken condition gives another -2 vs. saves.

Extra
3. Use a cruel weapon so that when you hit the shaken creature again, it also becomes sickened (another -2 to saves). Note- Blooded arcane strike/hurtful could get you that second attack in a single turn of move/attack. Bloodrager only, obviously.

This build allows you to hit enemies in the head so that they forget how to perform saves. Pair with a save or suck caster, and enemies would be hard pressed to resist.

A Cavalier who is a member of the Order of the Staff makes enemies more vulnerable to spells when he challenges them. Pair this with a character like a Sorcerer whose bloodline arcana makes them good at casting curses or compulsions, and you’ve got a team that specializes in shutting down particularly strong enemies.

I'm not trying to dissuade you from teamwork type of playing...However, I personally haven't seen much teamwork that meshed well (as opposed to mediocre meshing or not meshing at all). This is partially because I spent quite some time dealing with a slew of much lesser players who simply didn't get it.

RANT:
The big problem I have seen with teamwork in general has been the mix of characters in the party. Sure, everyone works towards the common good (or at least we hope they do), but everyone is on their own build path.

RAVE:
Not such a problem if you have a bunch of fighter types who can benefit from taking the extra feats. But this can become a massive problem with classes who don't get as many feats and therefore concentrate on feats that enhance the build.

CONCLUSION
(Edit)
In my opinion, we too often get caught up in optimizing what we see on our individual character sheet and lose track of what may or may not be helpful to the party as a whole.

I really like Shield Slam + Greater Bull Rush + Paired Opportunist. The idea is that With Shield Slam, you get a free Bull Rush with every hit. With Greater Bull Rush, your allies all get Attacks of Opportunity whenever you Bull Rush them out of Threatened Squares, and with Shield Slam, this also works with Flanking Buddies. With Paired Opportunist, you get the AoO as well, and that can be another Shield Bash, triggering more Attacks of Opportunity looping for as long as your Combat Reflexes last.

Broken Wing Gambit + maybe Paired Opportunist? Whenever either of you are attacked, both of you get attacks of opportunity.

Long and short, two mages that specialize in different spells with either the fire, cold, acid, electricity, water, earth, or air descriptor combine their spells for extra effects. The baseline benefit to sacrificing a spell slot to an ally's spells is that the primary spell's DC increases by 1 and gets a +1 to overcome spell resistance. Also, if you have Spell Focus and your ally has Spell Focus, then your Spell Focus increases their DC, as well.

Example: I have Spell Focus (Conjuration) and use Acid Arrow to augment your Fireball and you have Spell Focus (Evocation). Your Fireball gets the +1 DC from Element Commixture AND the +1 DC from my Spell Focus IN ADDITION to your own Spell Focus.

That's just the beginning, though. Because I used an acid spell to augment your fire spell, your Fireball does its normal effects, AND this happens:

"Lava (Earth/Fire): The targets are splattered with bits of molten rock and take 1d6 points of fire damage. The targets must succeed at a Reflex save or catch fire (see Catching on Fire on page 444 of the Pathfinder RPG Core Rulebook).

Now, 1d6 plus catching on fire isn't that great. But, guess what? Commixed spells can't be counterspelled normally AND the secondary effect goes off EVEN IF THE PRIMARY EFFECT IS SAVED AGAINST. So, that Rogue with Evasion just totally avoided your Fireball, but STILL has to save to avoid taking 1d6 and catching on fire.

And Earth/Fire is the worst combination. Water/Fire turns your Fireball into untyped (though non-lethal) damage and gets around energy resistance AND it can blind the target for 1d4 rounds. Air/Fire turns it into a smoke ball that can nauseate and blind creatures 1d4; and even if they save the Fireball AND save the secondary effect, they're STILL blinded for a round.

A mesmerist can be an excellent debuffer just on class features & can use feats to be a meatshield. A cleric has a lot of mind-affecting spells to take advantage of the hypnotic stare, and with a longspear plus possible buffs can back the mesmerist up.

There's several ways to make your character a ridable creature. Be a centaur, play a druid or a shifter, whatever. And a sohei monk can give substantial buffs to their mount, by their own class feature and by prereq-free mounted combat feats.

Basic of basics, make 2 melee focused Ratfolk. Swarming racial ability lets them share a hex and get flanking for their trouble.

An interesting idea is one player takes a Void Wizard and the other player takes someone that can cast a save or die spell. Most BBGs will blow off a save or die spell, but with a negative equal to half the wizard's level to saves it suddenly swings the balance to the player's favor. Considering the Wizard can do this 3+ int mod times per day, its more likely the other character will run out of save or suck spells first.

Long and short, two mages that specialize in different spells with either the fire, cold, acid, electricity, water, earth, or air descriptor combine their spells for extra effects. The baseline benefit to sacrificing a spell slot to an ally's spells is that the primary spell's DC increases by 1 and gets a +1 to overcome spell resistance. Also, if you have Spell Focus and your ally has Spell Focus, then your Spell Focus increases their DC, as well.

Example: I have Spell Focus (Conjuration) and use Acid Arrow to augment your Fireball and you have Spell Focus (Evocation). Your Fireball gets the +1 DC from Element Commixture AND the +1 DC from my Spell Focus IN ADDITION to your own Spell Focus.

That's just the beginning, though. Because I used an acid spell to augment your fire spell, your Fireball does its normal effects, AND this happens:

"Lava (Earth/Fire): The targets are splattered with bits of molten rock and take 1d6 points of fire damage. The targets must succeed at a Reflex save or catch fire (see Catching on Fire on page 444 of the Pathfinder RPG Core Rulebook).

Now, 1d6 plus catching on fire isn't that great. But, guess what? Commixed spells can't be counterspelled normally AND the secondary effect goes off EVEN IF THE PRIMARY EFFECT IS SAVED AGAINST. So, that Rogue with Evasion just totally avoided your Fireball, but STILL has to save to avoid taking 1d6 and catching on fire.

And Earth/Fire is the worst combination. Water/Fire turns your Fireball into untyped (though non-lethal) damage and gets around energy resistance AND it can blind the target for 1d4 rounds. Air/Fire turns it into a smoke ball that can nauseate and blind creatures 1d4; and even if they save the Fireball AND save the secondary effect, they're STILL blinded for a round.

Another fun one is a Skald paired with a caster focused around summoning; the Skald wants to take a Linnorm Death Curse rage power. These are normally useless to player characters since they require someone to die for them to do anything, but in the hands of a Skald buffing summons then you are quite happy to make killing your summons a rather uncomfortable proposition for your enemies.

Great idea, except it does not work very often. The death curse has a Will negates save of DC 10 + 1/2 character’s level + character’s Charisma bonus. Since the power is on the monster, it uses the monster's stats. Summons generally have low HD, making that modifier small. You need to research monsters with high Chr to make them have a significant modifier. Augmenting the summons does nothing. Mass Eagle's Splendor, will help, but only for monsters already out.

Rage powers granted by the skald uses the Skald's modifiers to determine the effect. They still use their own HD though.

Rage Powers wrote:

... If the rage power’s effects depend on the skald’s ability modif ier (such as lesser spirit totemAPG), affected allies use the skald’s ability modif ier instead of their own for the purposes of this effect.

One of my personal favorites is Alchemist/Brewmaster + Swashbuckler with Divine Fighting Technique (Blade and Tankard) and a fighting Tankard. This allows the alchemist to make alcoholic extracts via levels in Brewmaster, which lets the swashbuckler get potion effects, improves dirty trick, and allows the swashbuckler to take advantage of feats like Druken Brawler. For Flavor I have to recommend the drinking buddy team work feat, which allows for both players to reroll 1d20 when they are both drunk. The swashbuckler should also take the accelerated drinker trait which will let them drink as a move action. The rest of your feats can be spread however you want, possibly focusing on dirty trick for the swashbuckler, and brewing and drinking feats for the alchemist, who is then focused on mutagens or bombs. For the swashbuckler there you'll likely want two weapon grace regardless of build.